UPDATE 1-Tennis-Sluggish Serena begins Madrid defence, Li ousted

MADRID, May 5 (Reuters) - World number one Serena Williams recovered from a lethargic start to her Madrid Open title defence to seal an ultimately comfortable 7-6 6-1 victory against teenage qualifier Yulia Putintseva of Kazakhstan in the first round on Sunday.

On the opening day of action for several top players at the premier clay event, Chinese fifth seed Li Na also started slowly at the futuristic Magic Box arena but the former French Open champion was unable to turn around her match against promising American Madison Keys.

Keys, ranked 63 in the world and like Putintseva just 18 years of age, recorded her first victory against a top-10 opponent when she broke an erratic Li five times to win 6-3 6-2 and set up a second-round meeting with Spaniard Anabel Medina Garrigues.

Keys had lost in the final round of qualifying on Saturday but made it into the main draw as a lucky loser following the late withdrawal of Tamara Paszek and only found out she was playing Li about 10 minutes before she had to be on court.

"I just kind of started playing and felt really good and I was able to win," said Keys, who was doing her algebra homework in the players' lounge when she got the message she had a place in the main draw.

"Not overthinking it all day and sitting around and kind of worrying about it I think really helped," she added.

Chasing her fourth title of the year, Williams was tested by the 88th-ranked Putintseva early in the match but her superior power told in the end against a woman 13 years her junior.

Williams is making a fourth appearance in Madrid after beating Belarussian Victoria Azarenka in last year's final.

The American 15-times grand slam singles champion will meet Lourdes Dominguez Lino in the second round after the Spaniard beat fellow wildcard Simona Halep of Romania on Saturday.

"I felt a little sluggish out there, but I'm happy I was able to win the match," Williams told a news conference.

"I obviously have to and want to play better as each match goes on," added the 31-year-old.

Former world number one Caroline Wozniacki, the 10th seed, was upset 6-2 6-4 by unseeded Kazakh Yaroslava Shvedova.

Wozniacki, Li and Serena's sister Venus Williams, who withdrew from the tournament with a back injury on Sunday, were all in the top half of the draw and their exit potentially eases Serena's passage.

Sixth-seeded German Angelique Kerber had to come from a set down to get past unseeded Hsieh Su-wei of Taiwan, coming through 3-6 6-3 6-2 to set up a second-round clash against Alize Cornet of France.